Spotting the moment when Sainsbury's slips up has been a popular guessing-game among retail-watchers. The wait goes on. Wednesday's trading numbers were breezier than forecast – like-for-like sales, excluding fuel, were up 2.6% in the past three months. The share price, which has recently spent time below 300p on the thought that a wounded Tesco would take a swing at Sainsbury's, rose 4.5% to 319p.

As it happens, Tesco may not present the greatest medium-term headache. Justin King, Sainsbury's chief executive, took a risk when he adopted a price-match policy on branded goods but the strategy seems to have blunted the market leader's biggest weapon. The real fight is instead being conducted in "softer" areas, such as marketing and service, in which Sainsbury's currently looks better equipped. No, the greater financial threat is probably the one that faces the entire supermarket sector: too many shops and too much new space under construction. There's a classic dilemma here – over-capacity will hurt everybody but, since new stores are still the quickest way to reach new customers (yes, even in the age of home deliveries), nobody wants to be the first to stop building.

It's really a question of relative pain. Who is most at risk of cannibalising their own sales via expansion? On that score, Sainsbury's looks better placed than most since it is under-represented in the north and Scotland, where its opening programme is concentrated. To repeat, it's only a relative advantage – but it's worth something.